Hydrography within the Central and East Basins of the Bransfield Strait, Antarctica

The hydrography in the central and east basins of the Bransfield Strait is examined using data collected along a track 340 km long through the strait in November 1995, in conjunction with historical data from the NODC database. Circumpolar Deep Water (CDW) was seen throughout most of the central basin and the western east basin, in contrast to earlier studies that have shown CDW being absent or only weakly present in the strait. In the western part of the central basin the CDW was present as a narrow band near Deception Island with strong thermal gradients at its southern boundary, consistent with previous observations. Local deep-water formation produces bottom waters in the central and east basins that are distinct from one another and distinct from the surrounding deep waters outside the strait. The east basin has a more complex stratification than the central basin that must be the result of multiple sources. A model presented here explains the deep water in the east basin as a mixture between the sill waters and deep waters from both the central basin and the Weddell Sea. Weddell Sea sill water is the predominate end-member. Analysis of the historic data shows that temperature and salinity within the central and east basins of the Bransfield have varied considerably over the past 30 years. A systematic change in the slope of the T–S relation in the central basin is observed, the result of the middepth water becoming colder and fresher. The east basin has also become colder and fresher over the past 30 years; however, this change has occurred uniformly throughout the water column without affecting the slope of the east basin T–S relation. The changes in the east basin have been sporadic, with the largest change occurring between 1963 and 197

Increased dissolved terrestrial input to the coastal ocean during the last deglaciation

Here we present the first downcore results for a new paleoproxy, the Mn/Ca ratio of foraminiferal calcite, applied to sediment accumulated in the extreme Eastern Tropical North Pacific (ETNP) over the last 30,000 years. The Mn/Ca results are compared to oxygen isotopes and sea surface temperature calculated from Mg/Ca. We determined metal ratios using flow‐through time‐resolved analysis to minimize the effects of secondary mineralization. The foraminiferal species used for this study calcify at different depths. Core top ratios of these variant species change in proportion to the concentration of dissolved manganese in the water column at the depth of calcification. Since terrestrial input and oxidation reduction reactions control the levels of dissolved Mn in the oceans today, it therefore should be possible to use the Mn/Ca ratios of foraminifera as a proxy for these processes in the past. Mn/Ca of a mixed‐layer species (G. ruber) suggest that dissolved terrestrial input to the surface waters of the ETNP during the last glacial maximum was lower than today but began to increase with initial sea level rise and reached a maximum at 15 ka B.P. before coming down to present‐day levels at the end of sea level rise in the mid‐Holocene (7–5 ka). Ratios of a deeper calcifying species (N. dutertrei) mimic those of G. ruber over this same time period, consistent with shoaling of the 18°C thermocline. Mn/Ca of a benthic species (U. peregrina) does not show a maximum at 15 ka, suggesting that Mn was efficiently remineralized in the water column during deglaciation. Assuming that the period from the last glacial until the mid‐Holocene was a time of increased productivity, as elevated Mn might imply, the oxygen minimum zone (OMZ) was at least as well developed during deglaciation as it is today. Expansion of the OMZ may have contributed to the Mn/Ca trends we observe through time.Keywords: manganese, paleoprox

The rainbow hydrothermal plume, 36°15'N, MAR

The Rainbow hydrothermal plume was discovered during a recent geophysical survey along 200km of the Mid-Atlantic Ridge (MAR), SW of the Azores Triple Junction, in which at least seven new sites of hydrothermal activity were identified. Here, we present the first hydrographic study of the Rainbow plume, 36° 15'N, the strongest of the features located during that survey. The plume is detectable from real-time in situ nephelometer anomalies and extends 10-15km, W-E, in a non-transform discontinuity (NTD) between two adjacent ridge-segments. Maximum anomalies in the Rainbow plume indicate particle enrichments at least as large as those seen directly above the Trans Atlantic Geotraverse (TAG) vent-field (26°N, MAR). Analysis of hydrographic data indicates a vent source at Rainbow with a thermal output of up to 98 MW, representing 140% of the thermal output previously attributed to TAG. Both lines of evidence indicate the Rainbow plume to be the strongest such feature yet found on the MAR

Linking the 8.2 ka Event and its Freshwater Forcing in the Labrador Sea

The 8.2 ka event was the last deglacial abrupt climate event. A reduction in the Atlantic meridional overturning circulation (AMOC) attributed to the drainage of glacial Lake Agassiz may have caused the event, but the freshwater signature of Lake Agassiz discharge has yet to be identified in (delta)18O of foraminiferal calcite records from the Labrador Sea, calling into question the connection between freshwater discharge to the North Atlantic and AMOC strength. Using Mg/Ca-paleothermometry, we demonstrate that approx. 3 C of near-surface ocean cooling masked an 1.0 % decrease in western Labrador Sea (delta)18O of seawater concurrent with Lake Agassiz drainage. Comparison with North Atlantic (delta)18O of seawater records shows that the freshwater discharge was transported to regions of deep-water formation where it could perturb AMOC and force the 8.2 ka event

Barium cycling in the North Pacific: Implications for the utility of Ba as a paleoproductivity and paleoalkalinity proxy

Benthic incubation chambers have been deployed in a variety of geochemical environments that provide a comprehensive geochemical framework from which to address issues related to Ba geochemistry and the use of Ba as a paleoproxy. First order budgets for barium show that in the equatorial Pacific, present rates of Ba rain and benthic remobilization are nearly in balance, indicating that the rate of net accumulation is negligible and is clearly much less than the average for the Holocene; thus any paleoproxy algorithms built on the assumption of steady state are questionable. In contrast, budgets for sediments in the southern California Borderland indicate much higher burial efficiencies, in the range of 50–80%. The Ba:alkalinity (Alk) flux ratio is found to be remarkably constant throughout the environments studied and is indistinguishable from the deep water ratio used for paleoceanographic reconstructions. However, the Ba:organic carbon remobilization ratio is not constant. Combined, these results do not indicate a simple, first‐order direct link between Ba and alkalinity remobilization via organic carbon oxidation; however, the similarities in the Ba and alkalinity source functions conspire to maintain the Ba:Alk ratio near the global water column average. This latter observation provides promise for the use of the Ba:Ca ratio in benthic foraminifera as a paleocirculation tracer

Seawater intrusion through the oceanic crust and carbonate sediment in the Equatorial Pacific: Lithium abundance and isotopic evidence

Large-scale lateral advection of seawater within the oceanic crust is thought to be the cause of low heat flow in the Equatorial Pacific. Until now supportive evidence is limited to reversals of B and δ¹¹B, Ca, Mg, SO₄⁻², Sr and ⁸⁷Sr/⁸⁶Sr in sediment pore waters. Here we report new evidence from a detailed study of Li and its isotopes in sediment cores from ODP Sites 844 and 851. Carbonates at these sites were significantly recrystallized leading to large variations in Li and δ⁷Li in the sediments and associated pore waters. In addition to diagenetic effects, distinct reversals in lithium concentration and isotopic ratio toward modern seawater composition are observed in waters near the basaltic basement, lending further support to the seawater intrusion hypothesis. Three endmembers are identified in the pore waters: seawater, a diagenetically altered component, and evolved crustal fluid. The new lithium isotopic results underscore the importance of diagenetic artifacts in carbonate sediments

Eastern Pacific Warm Pool paleosalinity and climate variability: 0–30 kyr

Multiproxy geologic records of δ18O and Mg/Ca in fossil foraminifera from sediments under the Eastern Pacific Warm Pool (EPWP) region west of Central America document variations in upper ocean temperature, pycnocline strength, and salinity (i.e., net precipitation) over the past 30 kyr. Although evident in the paleotemperature record, there is no glacial-interglacial difference in paleosalinity, suggesting that tropical hydrologic changes do not respond passively to high-latitude ice sheets and oceans. Millennial variations in paleosalinity with amplitudes as high as ~4 practical salinity units occur with a dominant period of ~3–5 ky during the glacial/deglacial interval and ~1.0–1.5 ky during the Holocene. The amplitude of the EPWP paleosalinity changes greatly exceeds that of published Caribbean and western tropical Pacific paleosalinity records. EPWP paleosalinity changes correspond to millennial-scale climate changes in the surface and deep Atlantic and the high northern latitudes, with generally higher (lower) paleosalinity during cold (warm) events. In addition to Intertropical Convergence Zone (ITCZ) dynamics, which play an important role in tropical hydrologic variability, changes in Atlantic-Pacific moisture transport, which is closely linked to ITCZ dynamics, may also contribute to hydrologic variations in the EPWP. Calculations of interbasin salinity average and interbasin salinity contrast between the EPWP and the Caribbean help differentiate long-term changes in mean ITCZ position and Atlantic-Pacific moisture transport, respectively

Evidence of a dissolution effect on benthic foraminiferal shell chemistry: δ¹³C, Cd/Ca, Ba/Ca, and Sr/Ca results from the Ontong Java Plateau

Core-top benthic foraminifera (Cibicidoides wuellerstorfi) from a depth transect of Soutar box cores from the Ontong-Java Plateau (1.6 – 4.4 km) were analyzed for cadmium, barium, and strontium (Cd/Ca, Ba/Ca, and Sr/Ca) and for their stable isotopic composition (δ¹³C and δ¹⁸O). We also measure bottom water δ¹³C, Cd, and Ba at these sites. Foraminiferal δ¹³C values remain roughly constant over the entire depth range while bottom water δ¹³C values increase slightly, such that the δ¹³C difference between C. wuellerstorfi and bottom water ranges from about +0.2 ‰ in cores above 2.5 km to about -0.2 ‰ in cores below 4 km. This apparent depth dependence has not been previously reported, but this range in Δδ¹³C values is comparable to the uncertainty in published δ¹³C calibration studies We observe strong decreases in foraminiferal Cd/Ca, Ba/Ca, and Sr/Ca ratios (50, 25, and 15 percent, respectively) at water depths greater than about 2.5 km. These decreases are substantially larger than the corresponding changes in bottom water trace element concentrations, and they are not correlated with variations in pore water Cd and Ba concentrations at these sites. Together, the foraminiferal and bottom water Cd/Ca, Ba/Ca, and Sr/Ca data yield decreases in the apparent distribution coefficients for these metals into calcite with increasing water depth, again a pattern which has not been previously reported. These results when combined with the data from published core-top calibration studies suggest that a preferential loss of Cd, Ba, and Sr occurs during the dissolution of benthic foraminiferal calcite on the sea floor and raise the possibility of a dissolution-driven decrease in benthic foraminiferal δ¹³C values

Hydrography above the Mid-Atlantic Ridge (33°-40°N) and within the Lucky Strike segment

As part of the French-American Ridge Atlantic program the French-American Zero-Angle Photon Spectrometer and Rocks (FAZAR) cruise conducted water column studies between 33 and 40°N along the Mid-Atlantic Ridge (MAR) to detect hydrothermal activity and map its influence. This paper describes the large-scale hydrography within the axial valley, with particular emphasis on the hydrothermally active Lucky Strike segment (37°17’N). The FAZAR study area is affected by the presence of the Azores Current and Mediterranean Water (MW). Although the MW core has been mapped as far north as 50°N off the ridge, the northern boundary of the MW within the MAR in the FAZAR study area exists as a strong front south of the Azores platform. This front is most likely caused by the shallower ridge crest becoming a physical barrier to the MW. The Lucky Strike segment lies within this front and, as a result, has complicated hydrography which can obscure hydrothermal temperature and salinity anomalies

The geochemistry of Atlantic hydrothermal particles

Particles were collected from the dilute portion of neutrally buoyant hydrothermal plumes from four Mid-Atlantic Ridge sites (MARK, 23°N; TAG, 26°N; Broken Spur, 29°N; Lucky Strike, 37°N). Comparison of data from proximal portions of the TAG (Atlantic) [German et al., 1991; this study] and North Cleft (Pacific) [Freely et al., 1994] plumes show that oxyanion (e.g., V) is more efficient at TAG, possibly due to a higher proportion of Fe removed as sulfides at North Cleft and/or the more vigorous mixing in the high energy TAG buoyant plume. Chalcophile elements (e.g., Cu) show two stage removal. They are precipitated as sulfides during initial mixing of vent fluids with seawater and are sedimented from the buoyant plume. In the dilute plume they are scavenged from seawater by Fe oxyhydroxides. The REE show continued scavenging in the neutrally buoyant plume and lower levels in 1993 samples compared to 1988 samples [German et al., 1990] suggesting that the amount of scavenging is related to particle recycling.Copyrighted by American Geophysical Union